Impedance matched ultrasonic delay line wherein electrodes consist of bismuth and indium

ABSTRACT

A sonic delay line wherein electric impulses are converted to acoustic impulses passed through glass and reconverted to electrical impulses, wherein the acoustic impedance of the transducer is matched to that of glass utilizing an alloy of approximately equal parts of bismuth and indium, by weight.

ilu l a e ilnited Sta,

SEAR Cl-I R o-0 Boblett 1 Nov. 7, 1972 [54] IMPEDANCEMATCHED ULTRASONIC[56] References Cited DELAY LINE WHEREIN ELECTRODES CONSIST 0F BISMUTHAND INDIUM UNITED STATES PATENTS I t E v B u L Krause [72] or 2 0 e 0s 83,252,722 5/1966 Allen ..333/30 3,517,345 6/1970 Krause ..333/30 [73]Assignee: Ampex Corporation, Redwood City,

Cahf- Primary Examiner-Paul L. Gensler 22 Filed: Feb. 16, 1971Attorney-Robe" Clay [21] Appl. No.: 115,219 7 v ABSTRACT I sonic delayline wherein electric impulses are con- [52] US. Cl ..333/30 R, 310/8,333/32, verted to acoustic impulses passed through glass and 340/8 MMreconvened to electrical impulses, wherein the Cl -H03h 9/30 H04r17/007/38 acoustic impedance of the transducer is matched to [58] Flew Search"333/30 R1 32; 29/4731 that of glass utilizing an alloy of approximatelyequal parts of bismuth and indium, by weight.

5 Claims, 2 Drawing Figures PATENTED 7 1973 3. 702,448

I P s I l g N UT I OUTPUT INVENTOR.

EM/L V. BOBLETT F113- .2. W

ATTORNE V IMPEDANCE MATCHED ULTRASONIC DELAY LINE WHEREIN ELECTRODESCONSIST OF BISMUTH AND INDIUM SUMMARY OF THE INVENTION In theconstruction of bulk ultrasonic delay lines it is important to avoidreflections which result in unwanted echoes. In other words, althoughthere may be a satisfactory delay of the main signal component, if asmall portion of the signal is reflected back to the sender and againreflected back to the receiver, an echo will be produced which may beunacceptable in magnitude. In accordance with the present invention anacoustic matching system is employed in a delay line so that the thirdpath signal (i.e., the signal which has been reflected first by thereceiver and then again by the sender) is more than 60 dB down.Obviously the fifth and higher order path signals are even more greatlyattenuated. I

In the past, such sonic delay lines have caused multiple echoes and anunacceptable degradation of the main signal. i

In general the objects of the present invention are achieved byemploying a zero temperature coefficient glass together with an AC cutquartz crystal which matches the acoustic impedance of the glass withelectrodes on the quartz crystals which consist of essentially equalparts of bismuth and indium. Thus'the glass, transducer and electrodesare all carefully matched in acoustic impedance so that there is amaximum transmission of the desired signal and a maximum attenuation ofthe reflected signal.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view of an ultrasonicdelay line embodying the present invention.

FIG. 2 is a perspective view in section on an enlarged scale of thedelay line shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing byreference characters the delay element itself consists of a sheet ofglass 3 which preferably has a zero acoustic temperature coefficient.Mounted on each side of the glass are quartz transducers 5 and 7 andthese are preferably of the AC cut which operate in a shear mode.Although other quartz transducers might be used, these particular cutsgive the best sonic impedance match with the glass. Mounted on each sideof the glass are thin foils 9 and 13 of a bismuth-indium alloy. Thesethin foils serve as one of the electrodes on each of the quartztransducers and, by the selection of this particular alloy, make analmost perfect match between the glass and the quartz.

Mounted on the outside of each of the quartz plates is a relativelyheavy block of metal 15 and 17. These metal blocks are also made of thesame indium-bismuth alloy and serve as the outer electrodes for thequartz crystal and also, because of their composition and relativelylarge size, absorb any signal which might be reflected into them. Itwill be understood, of course, that the second path signal does notcause any problem if it can be effectively absorbed and prevented frombecom min th'd th' l.Th' t li couldb mea n of t he lz lmd ll t rs l9 t ll 3 5nd b o k II while the output signal is coupled by means ofconnectors 21 to the foil 9 and block 15.

The various parts can be cemented together and it is not necessary thatthe cement be matched acoustically since it can be made so thin that itis acoustically invisible. In one practical embodiment of the invention,an epoxy resin was used to cement the parts together but the thicknessof the resin was only from 5 to 10 microinches which is two to threeorders of magnitude smaller than the acoustic wavelength.

In one practical embodiment of the invention a delay line having a lmicrosecond delay was made utilizing a glass layer 0.1 inches thick. Thefoil layer on each side of the glass was I mil in thickness while thecrystals were 1.2 mils in thickness. The outer blocks were onesixteenthinch in thickness. This structure had an absorption less than 1.3 dB perinch at 20 MHz and the third path reflection signal was more than 60 dBdown.

Various glasses and crystal configurations can be used without departingfrom the spirit of this invention but it is preferred that the glasshave a zero acoustic temperature coefficient and that the crystals bequartz with an AC cut. However other glasses and other piezoelectricelements can be employed. The alloy itself, both for the foil and forthe blocks can vary from 48 to 52 percent bismuth and 52 to 48 percentindium.

I claim:

1. An ultrasonic delay line which includes a piece of glass as a sonicdelay element with transducers on each side of the glass, saidtransducers consisting of piezoelectric crystals with electrodesthereon, wherein the improvement comprises electrodes of an alloyconsisting of about equal parts of bismuth and indium, by weight.

2. The structure of claim 1 wherein the glass is temperature coefficientglass.

3. The structure of claim 1 wherein the electrodes between the crystalsand the glass consist of a thin film of said bismuth-indium alloy whilethe outer electrodes consist of thick blocks of bismuth-indium alloy.

4. The structure of claim 1 wherein the crystals are quartz, have an ACcut and operate in a shear mode.

5. The structure of claim 1 wherein the electrodes are from 48 to 52percent bismuth and 52 -48 percent indium, by weight.

* l t II!

2. The structure of claim 1 wherein the glass is a zero temperaturecoefficient glass.
 3. The structure of claim 1 wherein the electrodesbetween the crystals and the glass consist of a thin film of saidbismuth-indium alloy while the outer electrodes consist of thick blocksof bismuth-indium alloy.
 4. The structure of claim 1 wherein thecrystals are quartz, have an AC cut and operate in a shear mode.
 5. Thestructure of claim 1 wherein the electrodes are from 48 to 52 percentbismuth and 52 -48 percent indium, by weight.